In a known method for the continuous measurement of blood gases in vivo, use is made of an intravascular probe in the form of a flexible catheter having its distal end closed by a gas-permeable membrane. The said end of the probe is inserted into the blood vessel in question and its other end is connected to the inlet of a mass spectrometer whereby the device is evacuated. Gases absorbed in the blood diffuse through the membrane and pass along the catheter to the mass spectrometer wherein they are analysed.
U.S. Pat. No. 3,658,053 describes a blood catheter for use in the determination of the amount and type of dissolved gas in blood, which catheter includes a cannula of plastics material closed at one end. The cannula has an aperture in its wall towards said closed end, the exterior surface of at least that portion of the tube which includes the aperture being sheathed by a layer of gas-permeable material such as silicone rubber. Gases diffuse through the silicone rubber membrane and into the cannula via the aperture.
A constraint which has hitherto limited the practicability of known probes used in this procedure has been the need to manufacture the gas-permeable membrane from a bio-compatible material. Thus one known form of probe employs a flexible nylon catheter with a membrane of silicone rubber while another employs a malleable stainless steel catheter with a membrane of polytetrafluorethylene (PTFE). However, a disadvantage of silicone rubber as a membrane material is its inherently high gas permeability (typically in the region of 200.times.10.sup.-10 cm.sup.2 s.sup.-1 (cm Hg).sup.-1 for oxygen at 20.degree. C.) and the problem with a high permeability membrane is the tendency for the sampling region to become depleted of absorbed gas if the rate of transport of gas to the probe tip is not sufficiently high.
In other words the signal obtained from a probe of this type is undesirably dependent upon blood flow velocity. PTFE, on the other hand, is a virtually ideal membrane material from the standpoint of its inherent permeability. However, the problem with this material is the high temperature required for it to be worked, which precludes its use with flexible polymeric catheters. Thus, a PTFE membrane is limited to use with a catheter made from a material such as stainless steel, which does not however exhibit the same degree of flexibility as nylon for example. In particular the flexibility of stainless steel catheters is not sufficient to permit the safe monitoring of blood gas levels in infants.
Accordingly it is an aim of the invention to provide a form of construction for an intravascular probe or like device for use in the analysis of absorbed gases in liquids, whereby the above-discussed problems can be avoided.